Microscopic stripe formation and adhesion force increase introduced by local shear-stress deformation of polypropylene film

Localized shear deformation of biaxially oriented polypropylene (PP) film using a stylus force of 0.4 mN results in a reorientation of polymer strands in the shear-force direction, which appears as a striped region. This change in surface structure, which was observed by atomic force microscopy (AFM), resulted in an increase in the adhesion force as determined from the interaction between the AFM tip and the sample surface. This increase in adhesion force results from an increase in surface energy, which is thought to be caused by an increase in density and closer ordering of the polymer strands on the film surface. We demonstrate that the surface energy of PP film can be increased either by this mechanical scratching on the surface or by adding oxidation-induced chemical functional groups to the surface. The increase in surface energy from mechanical scratching of the PP surface is comparable to that introduced by surface oxidation of the unscratched PP surface after 1-min UV/ozone exposure.